Fischera, JoergDopita, Michael2015-12-100004-6361http://hdl.handle.net/1885/49606Aims. The aim of the paper is to give a physical explanation of the absence of the feature in the Calzetti extinction curve. Methods.We analyze the dust attenuation of a homogeneous source seen through a distant inhomogeneous distant screen. The inhomogeneities are described through an idealized isothermal turbulent medium where the probability distribution function (PDF) of the column density is log-normal. In addition it is assumed that below a certain critical column density the carriers of the extinction bump at 2175 Å are being destroyed by the ambient UV radiation field. Results. Turbulence is found to be a natural explanation not only of the flatter curvature of the Calzetti extinction curve but also of the missing bump provided the critical column density is NH = 1021 cm-2. The density contrast needed to explain both characteristics is well consistent with the Mach number of the cold neutral medium of our own Galaxy which suggests a density contrast s.σp≈6.Author/s retain copyrightKeywords: A-density; Cold neutral mediums; Column density; Dust , extinction; Extinction curves; Inhomogeneities; ISM: structure; Log-normal; Turbulent medium; Distribution functions; Dust; Mach number; Probability density function; Turbulence; Ultraviolet radiatio Dust, extinction; ISM: structure; Turbulence201110.1051/0004-6361/2011166442016-02-24